Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

David Rientjes
2020-01-18T23:36:10Z

On Sat, 18 Jan 2020, Andrew Morton wrote:

> On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang <richardw.yang@linux.intel.com> wrote:
>
> > If compound is true, this means it is a PMD mapped THP. Which implies
> > the page is not linked to any defer list. So the first code chunk will
> > not be executed.
> >
> > Also with this reason, it would not be proper to add this page to a
> > defer list. So the second code chunk is not correct.
> >
> > Based on this, we should remove the defer list related code.
> >
> > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> >
> > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> > Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: <stable@vger.kernel.org> [5.4+]
>
> This patch is identical to "mm: thp: grab the lock before manipulating
> defer list", which is rather confusing. Please let people know when
> this sort of thing is done.
>
> The earlier changelog mentioned a possible race condition. This
> changelog does not. In fact this changelog fails to provide any
> description of any userspace-visible runtime effects of the bug.
> Please send along such a description for inclusion, as always.
>

The locking concern that Wei was originally looking at is no longer an
issue because we determined that the code in question could simply be
removed.

I think the following can be added to the changelog:

----->o-----

When migrating memcg charges of thp memory, there are two possibilities:

(1) The underlying compound page is mapped by a pmd and thus does is not
on a deferred split queue (it's mapped), or

(2) The compound page is not mapped by a pmd and is awaiting split on a
deferred split queue.

The current charge migration implementation does *not* migrate charges for
thp memory on the deferred split queue, it only migrates charges for pages
that are mapped by a pmd.

Thus, to migrate charges, the underlying compound page cannot be on a
deferred split queue; no list manipulation needs to be done in
mem_cgroup_move_account().

With the current code, the underlying compound page is moved to the
deferred split queue of the memcg its memory is not charged to, so
susbequent reclaim will consider these pages for the wrong memcg. Remove
the deferred split queue handling in mem_cgroup_move_account() entirely.

----->o-----

Acked-by: David Rientjes <rientjes@google.com>